In the same way that dogs can't look up, the Viking can't defend itself from surface attack when on high, or air attack when grounded. Entire platoons can vanish if caught out.
This is deliberate, purposeful, clever balancing. The Blizzard team making Starcraft II are all long-term veterans of real-time strategy development. They're led by Rob Pardo - he worked as a designer on the original StarCraft, led the development of its expansion pack, Brood War, and now supervises design on all Blizzard's games. Following him is Dustin Browder - new to Blizzard, but a long-term employee of EA. He's credited as design lead on all of EA's strategy games since Red Alert, including C&C Generals and Battle for Middle-Earth.
It's a fascinating collaboration. Dustin explains how, when he joined the team, Rob laid down firm rules for how the sides would function. The main one: invention requires sacrifice. "The fundamental rule is that for every new unit we create for StarCraft II, we have to lose one from the roster." Lose one?
